PM Says Families To Get Help With The Cost Of Living

Families will get help with the cost of living, essential services will be guaranteed and road and rail projects will be rolled out in the budget, Prime Minister Malcolm Turnbull says.

Personal tax cuts for low and middle-income earners are expected to be a feature of Treasurer Scott Morrison's third budget on Tuesday, although it is unclear what form they will take.

"(It's) a budget that will ensure hard-working Australians can keep more of the money they earn," the prime minister told cabinet in Canberra on Monday.

"A budget that will enable us to have the resources to keep Australians safe, and a budget that will ensure the government lives within its means.

"Because we don't to throw a mountain of debt onto the shoulders of our children and grand children."

Mr Turnbull on Monday announced a $400 million project to duplicate the remaining single-track section of Sydney's Port Botany rail line as part of a $75 billion road and rail infrastructure package.

He also flagged help for people dealing with the high cost of power bills.

The promise of personal income tax cuts has been flagged for months in what could be the final budget before the next federal election.

Mr Morrison has conceded they won't be "mammoth" but says they will be affordable.

The Australian newspaper reported tax relief will come through increasing the low-income tax offset, while the budget promises tax cuts for higher income brackets by 2024.

At present workers with a taxable income below $66,667 get this offset.

Finance Minister Mathias Cormann also defended the coalition's decision to cap the amount of tax revenue collected at 23.9 per cent of gross domestic product limit.

Senator Cormann said the "speed-limit" on the public tax burden, above which revenue is returned through tax cuts, would keep the economy strong.

But public policy think-tank The Australia Institute describes the cap as "entirely arbitrary".

"There is no objective reason why the government should set this cap. The objective of fiscal policy should be whatever is appropriate for the state of the economy," institute executive director Ben Oquist said.

Shadow assistant treasurer Andrew Leigh questioned what is looking like a big spending budget based on a company tax revenue windfall.